Multi-stage pump assembly having a pressure controlled valve for controlling recirculation of fluid from the pump stage outlet to the pump stage inlet

ABSTRACT

Pump having a pump inlet, a pump outlet, at least two threaded rotors and a pressure controlled valve. The pressure controlled valve is capable of controlling re-circulation of fluid from the pump outlet to the pump inlet. The pressure controlled valve can be a control valve. A multiple stage pump assembly is also provided having least two pumps arranged in series, in which at least one of the pumps is the aforementioned pump.

This application is the U.S. national phase of International ApplicationNo. PCT/EP2012/059249 filed 18 May 2012 which designated the U.S. andclaims priority to European Patent Application No. 11250542.5 filed 20May 2011, the entire contents of each of which are hereby incorporatedby reference.

This invention relates to multiple stage rotary screw pump assemblies,particularly for use in wells such as oil and gas wells.

BACKGROUND OF THE INVENTION

In the oil and gas production industry, it is sometimes desirable to usepumps to assist in the production of fluids from a well. For example,there may be insufficient pressure in the formation around an older wellto lift the formation fluids to the surface. In another situation, aheavy fluid may be introduced into a well to stop formation fluidsflowing up the well. In order to put the well back into production, theheavy, “kill” fluid must be lifted from the well using a pump.

Rotary screw pumps, such as twin or triple screw pumps, are positivedisplacement pumps which use rotating screws to pressurise a fluid.Rotary screw pumps are known for their ability to pump multiphasefluids.

In addition, it is known that, to generate high differential pressures,a pump may be constructed with multiple pumping stages. The total pumpdifferential pressure is the sum of the individual stage differentialpressures. Similarly, compressors can be constructed with multiplecompressor stages, in order to generate high pressures in gases.Multiple stage pumps generally have pumping stages of equal swept volumewhereas multiple stage compressors generally have compression stages ofdecreasing swept volume. By swept volume it is meant, in the case of amultiple screw pump for example, the volume of fluid discharged from thestage discharge during one complete revolution of the screws. Thedistinction between multiple stage pumps and multiple stage compressorsarises since liquids are nearly incompressible whereas gases arecompressible.

A multiple stage twin screw pump is disclosed in U.S. Pat. No.6,413,065. This document proposes a multiple stage downhole pump havingmultiple twin screw pumping modules connected in series.

U.S. Pat. No. 7,093,665 discloses another downhole multiple stage twinscrew pump. This document discusses a problem with the pump assemblydescribed in U.S. Pat. No. 6,413,065. It is said that, in situationswhere there is a low liquid content and a high gas content in the fluid,the amount of liquid present is insufficient to effectively seal thegaps between the screw threads and the rotor housing. As a consequence,the pump cannot maintain the pressure difference across the pump and thepump loses efficiency.

U.S. Pat. No. 7,093,665 then discloses a method of adapting a downholepump such as the one described in U.S. Pat. No. 6,413,065 for use inwells having a high gas content. In one embodiment, a liquid trap and asupplementary liquid channel is provided to capture a portion of theliquid near the outlet of the multiple stage twin screw pump and returnit to the intake of the multiple stage twin screw pump. In this way, theliquid seal around the twin pumping screws can be enhanced.

A multistage pump assembly is also described in our pendingInternational patent application, WO2010/092320. In this assembly, aplurality of components are provided which comprise a plurality ofpre-assembled pump modules including at least one twin screw pumpmodule. An elongate sleeve for housing the components and securing meansattachable or engagable with a portion of the elongate sleeve are alsoprovided. The securing means are operable to fixedly retain thecomponents within the sleeve.

These pump arrangements do not address a further problem which ariseswhen pumps of this type are used to generate high pressures in amultiphase fluid, as is often desirable in oil and gas well pumpingapplications. Due to the compressibility of gas, the rate at which fluidis delivered from one pump stage to a subsequent pump stage in amultiple stage pump assembly is less than the rate at which thesubsequent pump tries to draw fluid into its intake. Accordingly, thelast pump stage starts “sucking” on the previous pump stages, and thepressure difference across the last pump stage increases. In fact, thepressure difference across the pump stages increases from the first tothe last pump.

A high proportion of the pressure generation occurs in the final stageof the pump. Consequently, this area of the pump can become extremelyhot, reducing running clearances and risking seizure. Accordingly, whenthe percentage of gas in the pumped fluid is high, a multiple stagerotary screw pump becomes very inefficient.

The prior art pumps do not address this issue and so can suffer from theproblems of over-heating and seizure caused by the final pump stageperforming most of the work when the pumped fluid is a multiphase fluid.

A multiple stage pump could be designed more like a compressor, with aprogressive reduction in the swept volume of its stages. Such a multiplestage pump would have its stages tailored for a particular gas to liquidratio. To illustrate this, consider an oil well producing a fluid at100° C. and having the following composition:

-   -   Oil: 2000 bbls/day (318 m³/day)    -   Water: 2000 bbls/day (318 m³/day)    -   Gas: 1000 bbls/day (159 m³/day).

Consider a four stage pump assembly with the following pressurerequirements:

-   -   Intake pressure: 1000 psig (6.89 MPa)    -   Discharge pressure: 3000 psig (20.7 MPa).

To share the work equally between the four stages of the pump assembly,each stage would need to pressurise the fluid by 500 psig (3.45 MPa)(ignoring the effect of fluid shrinkage on hydraulic hp). In order to doso, a multiple stage pump would have to have stages with the followingswept volumes:

Stage 1

-   -   Total intake volume: 5000 bbls/day (795 m³/day)    -   Assuming a negligible temperature rise through the pump, the        liquid is incompressible and the gas behaves as an ideal gas.        So, for the gas fraction:    -   Intake pressure=1000 psig (6.89 MPa)=1014.7 psia (7.00 MPa        absolute)    -   Intake gas volume=1000 bbls/day (159 m³/day)    -   Discharge pressure=1500 psig (10.3 MPa)=1514.7 psia (10.4 MPa        absolute),    -   Discharge gas volume=1014.7×1000/1514.7=669.9 bbls/day (107        m³/day)    -   Total discharge volume=4669.9 bbls/day (742 m³/day) (i.e. liquid        plus discharge gas)        Stage 2    -   Total intake volume=4669.6 bbls/day (742 m³/day)    -   Intake pressure=1500 psig (10.3 MPa)=1514.7 psia (10.4 MPa)    -   Intake gas volume=669.9 bbls/day (107 m³/day)    -   Discharge pressure=2000 psig (13.8 MPa)=2014.7 psia (13.9 MPa)    -   Discharge gas volume=1514.7×669.9/2014.7=503.6 bbls/day (80.1        m³/day)    -   Total discharge volume=4503.6 bbls/day (716 m³/day)        Stage 3    -   Total intake volume=4503.6 bbls/day (716 m³/day)    -   Intake pressure=2000 psig (13.8 MPa)=2014.7 psia (13.9 MPa)    -   Intake gas volume=503.6 bbls/day (80.1 m³/day)    -   Discharge pressure=2500 psig (17.2 MPa)=2514.7 psia (17.3 MPa)    -   Discharge gas volume=2014.7×503.6/2514.7=403.5 bbls/day (64.2        m³/day)    -   Total discharge volume=4403.5 bbls/day (700 m³/day)        Stage 4    -   Total intake volume=4403.5 bbls/day (700 m³/day)    -   Intake pressure=2500 psig (17.2 MPa)=2514.7 psia (17.3 MPa)    -   Intake gas volume=403.5 bbls/day (64.2 m³/day)    -   Discharge pressure=3000 psig (20.7 MPa)    -   Discharge gas volume=2514.7×403.5/3014.7=336.6 bbls/day (53.5        m³/day)    -   Total discharge volume=4336.6 bbls/day (689 m³/day)

Accordingly, for these well fluid and pumping conditions, a perfectlymatched pump would require rotor sets with the following swept volumes:

-   -   First stage: 5000.0 bbls/day (795 m³/day)    -   Second stage: 4669.6 bbls/day (742 m³/day)    -   Third stage: 4503.6 bbls/day (716 m³/day)    -   Fourth stage: 4403.5 bbls/day (700 m³/day)

In this example the gas constitutes only 20% of the total fluid volumeinto the pump intake and the pressure rise is relatively modest, but thedifference in ideal swept rotor volume is greater than 10% between thefirst and last stage. This highlights the significant impact that thegas to liquid ratio can have.

However, there is a significant problem with multiple stage pumpassemblies having decreasing swept volumes for the pump stages, in that,if the well fluid gas to liquid ratio changes, the pump stages quicklybecome mismatched with the gas to liquid ratio. If the volume of gasincreases, each stage throughout the pump attempts to draw more fluidthan the preceding stages can deliver. The later stages effectively suckon the preceding stages and the preceding stages can thereforecontribute little effective work. This is the same scenario as describedabove for a constant volume multiple stage pump. If, on the other hand,the volume of gas decreases, the fluid volume discharged from an initialstage would be higher than that scavenged by a subsequent stage. Thepressure of the fluid between the stages would rise rapidly, causing thepump to hydraulically lock or burst the housing or seals.

When pumping fluids from subterranean hydrocarbon bearing formations,this problem associated with multiple stage pumps used to pressurisemultiphase fluids is particularly hard to address because thehydrocarbon liquids are volatile, containing gas in solution, and,depending upon the pressure of the reservoir, may further contain aproportion of free gas. Indeed a hydrocarbon reservoir may produce oilas a liquid initially but, as production continues and the pressure ofthe reservoir falls below the “bubble point”, will later flow a mixtureof oil and gas. Every oilfield and every well within a field will haveunique properties, depending on the hydrocarbon fluids themselves andthe pressure of the fluids at that spatial and chronological point inthe reservoir. To match the swept volume of successive stages in a pumpto the fluid properties of an individual well at a given point in timewould require an almost infinite number of rotor sizes and animpractical number of well interventions to change the pump to one moresuited to the current conditions.

U.S. Pat. No. 5,779,451 describes the problems encountered when aconventional single rotary screw pump is used to pump fluids having ahigh gas fraction. It explains that overheating and seizure can occurdue to lack of cooling liquid and a greater amount of heat generationacross the last thread of the screw. The document teaches an improvedtwin-screw pump for providing a large pressure boost to highgas-fraction inlet streams. The pump includes a housing having aninternal rotor enclosure, the rotor enclosure having an inlet and anoutlet and a plurality of rotors operably contained in the enclosure.Each rotor has a shaft and a plurality of threads affixed thereon, therotors being shaped to provide a non-uniform volumetric delivery ratealong the length of each rotor. In one embodiment, the rotors have aplurality of threaded pumping stages separated by unthreaded non-pumpingchambers. The threads of each pumping stage may have a different screwprofile to provide progressively decreasing inlet volumetric deliveryrates from the inlet to the outlet of the rotor enclosure. It is saidthat this arrangement can pump high gas to liquid ratio fluids withimproved power efficiency and without seizing.

The document further teaches modifications to allow the pump to pumpincompressible fluids. To accommodate incompressible fluids, each of theinter-stage chambers can be connected to the outlet of the pump and maybe connected to a pressure reservoir. So, excess liquid can be bled tothe outlet or the pressure reservoir. Check valves prevent back-flowfrom the outlet to the chambers. The connections between the chambersand the outlet can have pumps in them to drive fluid to the outlet.

GB 2299832 teaches a similar arrangement to that described in U.S. Pat.No. 5,779,451. Two sets of threads are provided on a single rotor in asingle pump housing. A bleed port with a pressure relief valve isprovided between the two sets of threads to relieve the spike of liquidvolume and pressure which occurs whenever the void fraction of thepumped fluid becomes zero. Bleed fluid may be discarded, drained to asump for recycling, re-circulated directly to the inlet of the pump, orhandled otherwise.

Neither of these disclosures addresses the problem of an unevendistribution of work in a multiple stage rotary screw pump, as discussedabove.

A contradiction therefore exists, in that although a single stage rotaryscrew pump is well known to be useful for pumping multiphase fluids, amultiple stage screw pump is not well suited to pumping multiphasefluids because the work cannot be distributed evenly between the variousstages of the pump.

For these reasons, pumps used for hydrocarbon extraction typically areeither multiple stage centrifugal pumps, which do not fix the volumetriccapacity of each stage, or positive displacement pumps having a singlestage. This approach avoids the need to match the swept volume of thepump to the pumped fluid volumes at the conditions encountered at eachstage throughout the pump.

However, centrifugal and single stage pumps are not without theirproblems. Centrifugal pumps in particular are unable to process fluidswith a high percentage of free gas because the gas accumulates withinthe hubs of their impellors causing the pump to loose prime andcavitate, a condition commonly described as gas locking. Single stagerotary screw pumps cannot efficiently develop the high pressuresrequired to pump fluid from deep hydrocarbon bearing formations.Accordingly, to date, most twin screw multiphase pumps have been used insurface applications that require only a relatively low boost pressure.

There remains a need for a pump assembly which can be used more reliablyand efficiently to pump multiphase fluids.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a pump comprises apump inlet, a pump outlet, at least two threaded rotors and a pressurecontrolled valve, the pressure controlled valve being capable ofcontrolling re-circulation of fluid from the pump outlet to the pumpinlet. The threaded rotors can cause fluid to move from the pump inletto the pump outlet.

According to a second aspect of the invention, a multiple stage pumpassembly comprises at least two pumps arranged in series, at least oneof the pumps being the pump of the first aspect of the invention.

For a given pump, by re-circulating fluid (i.e. a proportion of thefluid pumped through the pump) from the pump outlet to the pump inletthrough the pressure controlled valve, the pressure difference acrossthe pump can be controlled. In accordance with the equation:hydraulic power=mass flowrate×pressure increase  (1)it can be seen that the power generated by that pump can consequently becontrolled, since the mass flowrate is fixed (assume a typical twinscrew pump with solid inflexible intermeshing rotors).

Where the pump is part of a multiple stage pump assembly, controllingthe power generated by a pump by re-circulating fluid from its outlet toits inlet can cause the differential pressure across the preceding pumpto increase. Accordingly, the power generated by that preceding pumpconsequently increases.

Accordingly, by re-circulating fluid from the pump outlet to the pumpinlet of some or all of the pumps in a multiple stage pump assembly, thework done by the multiple stage pump assembly can be distributed moreevenly across the pumps.

Re-circulating pressurised fluid from the pump outlet to the pump inletresults in some energy being sacrificed and so, on the face of it, itmay seem that the multiple stage pump assembly would be less efficient.The pump and multiple stage pump assembly of the invention may thereforeappear to be a retrograde step. However, it has been found that byre-circulating fluid as described above, an improved multiple stage pumpassembly can be made since higher pressures can be generated withoutoverloading the final pump in the assembly. Also, the reliability of themultiple stage pump assembly is increased markedly. This is because thepreceding pumps are forced to contribute more (and possibly equallydepending on the fluid composition) to the total work done by themultiple stage pump assembly, where conventionally they contributelittle. Thus the burden of work is shared between all the pumps in amultiple stage pump assembly.

Further, the design of individual components can be optimised for theloads imposed upon them because the work and loadings are controlled bythe pressure settings of the valves.

Preferably, the pressure controlled valve is capable of controlling therate of fluid flow there-through. The pressure controlled valve isadapted to control the rate of fluid flow in proportion to the gas toliquid ratio of the fluid.

The pressure controlled valve is preferably a control valve. As is wellknown in the art, control valves are valves which are designed tocontrol the flow of fluid by adjusting the degree to which the valve isopen anywhere from 100% closed to 100% open. Control valves canprogressively and continuously adjust the degree of opening of thevalve. In contrast, isolation valves (such as mushroom, gate, ball andflapper valves) are designed essentially as pressure relief valveswhereby the valve is either fully closed or fully opened. In the fullyopen position, isolation valves can quickly relieve pressure to adesired level at which point they return to the fully closed position.Isolation valves are not designed to control the opening of the valve toany degree between 100% closed and 100% open.

Examples of control valves are needle valves, sleeve valves andbutterfly valves. A needle valve has a tapered/conical needle which sitsinside and mates with a tapered/conical seat to close the valve. As theneedle is withdrawn from the seat, a flow path opens. The width of theflow path increases as the needle is withdrawn from the seat. A sleevevalve has two concentric sleeves which can move axially relative to oneanother. Each sleeve has an aperture and the extent of overlap of theapertures can be varied by relative axial movement of the sleeves. Oneof the apertures may have an increasing width so as to provide anincreasing rate of aperture overlap for a given amount of relative axialmovement.

An example of a suitable sleeve valve is depicted in FIG. 3. The outersleeve has a rectangular aperture. The inner sleeve (shown in brokenlines) sits inside the outer sleeve and has a curved aperture, resultingin a non-linear increase in flow rate as shown in FIG. 4. In otherwords, a low flow rate is permitted with initial overlap of theapertures but the flow rate increases rapidly as the pressure differenceand therefore overlap increases.

A control valve can avoid valve chatter and promote stability in thedeveloped pump inter-stage pressures. Accordingly, by using controlvalves, a multiple stage pump assembly comprising a plurality of pumpsin accordance with this invention can be very responsive and can quicklyreach an equilibrium state in which each valve is opened to anappropriate degree to optimise the distribution of work among thevarious pumps. Thus a steady state is achieved for the gas to liquidratio of the fluid being pumped.

Sleeve valves have the further benefit that they can be self-cleaning,which can be particularly useful in a well environment which may containsolid particles, such as sand.

The pump of the first aspect of the invention may further comprise aconduit connecting the pump outlet to the pump inlet. Re-circulatedfluid can flow through the conduit. The pressure controlled valve may beassociated with the conduit so as to selectively allow fluid to flowthrough the conduit from the pump outlet to the pump inlet. The pressurecontrolled valve may be located wholly or partly within the conduit oradjacent one or other end of the conduit.

In one embodiment, at least the second and each subsequent pump in amultiple stage pump assembly is in accordance with the first aspect ofthe invention. In this case, the first pump may or may not be inaccordance with the first aspect of the invention. The first pump isconsidered to be at the intake end (i.e. lowest pressure side) of themultiple stage pump assembly. It follows that the last pump isconsidered to be at the discharge end (i.e. highest pressure side) ofthe multiple stage pump assembly.

Such a multiple stage pump assembly can be used advantageously to pumpfluids with compositions varying from 100% liquid to a high gas toliquid ratio since liquid can be re-circulated from the pump outlet tothe pump inlet of the various pumps, thereby causing the work to bedistributed more evenly between the pumps.

Each of the pumps of the multiple stage pump assembly described abovemay have the same swept volume.

Alternatively, there can be a reduction in the swept volume of each pumpfrom the first pump to the last pump in the series (i.e. from an intakeend to a discharge end of the multiple stage pump assembly). Thisarrangement is also known as a ‘tapered’ pump assembly and is analogousto an arrangement for compressing a gas, as described above. Thereduction in swept volume along the series of pumps allows a multiplestage pump assembly to be tailored for optimum operation with aparticular fluid composition (i.e. a particular gas to liquid ratio),which it is expected that the multiple stage pump assembly willencounter. However, by providing valves to re-circulate fluid as set outabove, the tapered multiple stage pump assembly can also handleefficiently fluid compositions which vary from the particularcomposition expected.

For example, a tapered multiple stage pump assembly can operateeffectively for gas to liquid ratios which are greater than the gas toliquid ratio for which the taper is tailored by providing pumps inaccordance with the first aspect of the invention for at least thesecond and each subsequent pump in the series. It is anticipated thatthe first pump in the series can be a conventional pump, such as aconventional rotary screw pump. However, it may also be a pump inaccordance with the first aspect of the invention.

In another example, at least the penultimate pump and each precedingpump in a multiple stage pump assembly is in accordance with the firstaspect of the invention. This is particularly useful where there is areduction in swept volume from the first pump to the last pump in theseries. The last pump in the series may or may not be in accordance withthe first aspect of the invention. In this example, a gas to liquidratio can be handled which is less than the gas to liquid ratio forwhich the swept volumes of the multiple stage pump assembly have beentailored.

A particularly useful embodiment of the second aspect of the inventionis one in which all the pumps of the multiple stage pump assembly are inaccordance with the first aspect of the invention and wherein there is areduction in the swept volume of each pump from the first pump to thelast pump in the series. The taper of such a pump can betailored/optimised for the fluid composition which is likely to beencountered in use but, in the event that the fluid composition changes(either permanently or in the short term), the pump can also handle veryeffectively fluid compositions having both a higher and a lower gas toliquid ratio.

The pressure controlled valve can be one which responds to the absolutepressure difference between the pump outlet and the pump inlet. In otherwords, the valve permits fluid to flow there-through when the absolutepressure difference between the pump outlet and the pump inlet reaches athreshold level. The threshold level for activating the valve istypically approximately the same, though it may be different, for eachpump in a multiple stage pump assembly. In a preferred example, thethreshold level can be approximately equal to or just greater than theoverall boost pressure to be obtained by the multiple stage pumpassembly divided by the number of pumps in the multiple stage pumpassembly (i.e. the number of ‘stages’). By overall boost pressure, it ismeant the differential pressure across the multiple stage pump assembly.

Where each pump in a multiple stage pump assembly comprises such avalve, the overall pumping pressure which can be achieved by themultiple stage pump assembly will necessarily be limited by operation ofall of the pressure controlled valves. This may be circumvented by usinga conventional pump as the first pump in the series. Since fluid is notre-circulated around the first pump, the first pump will simply workharder as the gas to liquid ratio increases, thereby permitting agreater overall pumping pressure to be obtained.

Alternatively, the pressure controlled valve can be one which respondsto the ratio between the pressure at the pump outlet and the pressure atthe pump inlet. In other words, the valve permits fluid to flowthere-through when the ratio between the pressure at the pump outlet andthe pressure at the pump inlet reaches a threshold. This can be achievedusing a valve which comprises a piston having an inlet face and anoutlet face. In use, the inlet face is exposed to the pump inletpressure and the outlet face is exposed to pump outlet pressure. Thesurface area of the inlet face is greater than the surface area of theoutlet face and the ratio between the area of the inlet face to the areaof the outlet face prescribing the threshold ratio between the pressureat the pump outlet and the pressure at the pump inlet. With such anarrangement, even distribution of work across the pumps in a multiplestage pump assembly can be achieved without limiting the overall pumpingpressure that can be obtained by the assembly.

The threshold ratio between the pressure at the pump outlet and thepressure at the pump inlet can be different for each pump in theassembly. Typically, in order to distribute the work evenly between thepumps, the threshold ratio for pumps in a multiple stage pump assemblydecreases from the intake of the multiple state pump assembly to thedischarge of the multiple stage pump assembly.

For example, consider a multiple stage pump assembly having four pumpstages. For an intake pressure “4P”, to achieve a pressure rise perstage of “P”, the pressure ratio for the stages must be: 1.25:1 (firststage); 1.2:1 (second stage); 1.17:1 (third stage); 1.14:1 (fourthstage) (based on an intake pressure, 4P, an outlet pressure from thefirst pump of 5P, an outlet pressure from the second pump of 6P, anoutlet pressure from the third pump of 7P and an outlet pressure fromthe fourth pump of 8P).

This arrangement can be suitable where the bottom hole pressure (i.e.the pressure at the bottom of the well) and well productivity is knownwith reasonable accuracy, since the ratio for the/each pump is afunction of fluid properties and absolute pump intake pressure (in turnrelated to flowing bottom hole pressure).

In a particularly useful embodiment, the pressure controlled valve canbe one which responds to the ratio between the pressure differencebetween the outlet and the inlet of the pump stage (dP_(stage)) and thepressure difference between the discharge and intake of the overallmultiple stage pump assembly (dP_(assembly)). In other words, the valvepermits fluid to flow there-through when the ratio between dP_(stage)and dP_(assembly) reaches a threshold. This can be achieved using avalve which comprises a piston having an inlet face which is exposed inuse to the pump inlet pressure and an outlet face which is exposed inuse to the pump outlet pressure and two chambers, one of which is influid communication with the intake of the multiple stage pump assemblyand the other is in communication with the discharge of the multiplestage pump assembly, such that the pressures in the chambers whichcorrespond to the intake and discharge pressures of the multiple stagepump assembly oppose the inlet and outlet pressures of the pumprespectively. The ratio of the surface area of the inlet face or theoutlet face to the cross-sectional area of one of the chambersprescribes the threshold at which the valve will permit fluid flow.

With such an arrangement, it is possible to evenly distribute the workacross all the pumps without knowing what the bottom hole pressure is.

The valve may comprise an actuator and a valve element, the valveelement being that part of the valve which provides a fluid flow path,whereby the actuator can actuate the valve element to control fluid flowthrough the valve element. The actuator and the valve element may beintegral or may be remote. The pistons in the embodiments describedabove may form at least part of the actuator.

The valve of the multiple stage pump assembly may be a two-way valve sothat the pump assembly can operate in both directions. This can beuseful in pipeline pumping or Water Alternating Gas (WAG) injectionoperations, which is an enhanced oil recovery technique in which waterinjection and gas injection are alternated.

The valve does not need to be exactly matched to the expected fluidproperties because by making the valve opening pressure sensitive, thevolume of re-circulated fluid can be continuously variable.

The or each pump is preferably adapted to preferentially allow, in use,liquid to pass through the pressure controlled valve rather than gas. Inthis way, the fluid re-circulated from the pump outlet to the pump inletis primarily or entirely liquid, whereas most or all of the gas presentin the fluid is passed on to the next pump stage in the series. It hasbeen found that the reduction in efficiency caused by re-circulatingfluid already pressurised by one or more pumps is minimised if the fluidre-circulated is liquid, as opposed to a mixture of liquid and gas.

Preferentially allowing liquid rather than gas to pass through the valvein use can be achieved by way of a recess into which the liquid flowsunder gravity. For example, a recess can connect the outlet of the rotorenclosure in which the rotary screws are located to the conduit forre-circulating fluid.

The conduit may be formed as part of the pump. For example, it mayextend through the body or along the outside of the pump. Alternatively,the conduit may be separable from the pump such that it can be removablyconnected in fluid communication with the inlet and outlet of the pump.

According to a third aspect of the invention, a method of pumping afluid from a first location to a second location comprises providing amultiple stage pump assembly having two or more pumps in series whereinat least one of the pumps is adapted to re-circulate fluid from itsoutlet to its inlet, positioning an intake end of the multiple stagepump assembly at or near the first location, activating the multiplestage pump assembly to pump the fluid from the first location to thesecond location, and re-circulating fluid from the outlet to the inletof said at least one pump. Re-circulation of fluid can be controlled inproportion to the gas to liquid ratio of the fluid. The pump may be thepump of the first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a schematic of a multiple stage twin screw pump assembly;

FIG. 2 is a view from above of one of the pumps forming the multiplestage pump assembly of FIG. 1;

FIG. 3 is a schematic of a sleeve valve;

FIG. 4 is a chart showing flow rate against pressure difference for atypical sleeve valve as shown in FIG. 3;

FIG. 5 is a schematic of another valve which may be used in the pumpsshown in FIG. 1;

FIG. 6 is a schematic of yet another valve which may be used in thepumps shown in FIG. 1;

FIG. 7 is a schematic of a second embodiment of the invention;

FIG. 8 is a schematic of another embodiment of the invention;

FIG. 9 is a schematic of another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A multiple stage pump assembly 1 in accordance with the second aspect ofthe invention can be seen in FIG. 1. The multiple stage pump assembly 1is suitable for pumping a multiphase fluid in the direction marked byarrows “A”. It can be understood that this multiple stage pump assemblycould be used to lift fluids from a well.

The multiple stage pump assembly 1 comprises four pumps 2,3 in series.The first pump 2 in the series (“first” since it is at the intake end 4of the multiple stage pump assembly) is a conventional rotary screw pumpas known in the art. The second, third and fourth pumps 3 are inaccordance with the first aspect of the invention. The fourth pump isreferred to as the last one in the series as it is at the discharge end5 of the multiple stage pump assembly 1.

Each pump 2,3 has two threaded rotors 6 located in a rotor chamber 15for driving fluid from an inlet 7 to an outlet 8 of that particularpump. Although two rotors are depicted in FIG. 1 (i.e. a twin-screwarrangement), other numbers of rotors could be used instead, such asthree (triple screw arrangement) or more. Also, although FIG. 1 (andlater figures) depicts a single pair of rotors driving fluid in onedirection, it is possible for each pump to comprise opposing pairs ofrotors such that the fluid drawn into the inlet of each pump is splitinto two streams, each stream being driven through one of the pairs ofrotors and then re-combined before the outlet of the pump, as describedin U.S. Pat. No. 6,413,065.

It is known in multiple stage rotary screw pump assemblies to includeone or more additional units, such as units associated with each of thepump stages (e.g. between the pump stages). For example, these units mayinclude gear modules, spacer units, sealing units or plenum chambers andthe like. In this example, a single spacer unit 9 is depicted betweeneach pump which transfers the drive from one pump to the next, and agear module 10 is located at the discharge end 5 of the multiple stagepump assembly. Although not shown in detail, the spacer units 9 and thegear module 10 naturally have conduits 16 there-through to allow thepassage of fluid from one pump to the next. However, it may beunnecessary to provide any units between the pump stages depending onthe nature of the rotary screw pump. The precise design of the rotaryscrew pump and whether any associated units are required will beapparent to the person skilled in the art and is not the subject of thisinvention.

Each pump in accordance with the invention comprises a conduit 11 influid communication with the pump inlet 7 and the pump outlet 8.Specifically, one end 12 of the conduit 11 is open to the pump inlet 7and the other end 13 of the conduit 11 is open to the pump outlet 8. Asdepicted in FIG. 1 and in the top view shown in FIG. 2, channels 17 inthe end faces of the pump connect the pump inlet 7 and the pump outlet 8to the conduit 11.

A pressure controlled valve 14 is positioned in the conduit 11, althoughthe valve 14 could actually be located at or adjacent either end 12,13of the conduit 11. Ideally, and as shown, the entry to the valve 14 isarranged below the pump outlet 8 when the multiple stage pump assembly 1is arranged vertically, as in use.

The valve 14 is a sleeve valve as shown in FIG. 3. The sleeve valvecomprises an outer sleeve 18 and an inner sleeve 19 positionedco-axially within the outer sleeve 18. The outer sleeve 18 is formedwith a rectangular (ignoring the effect of the curvature of the sleeve)aperture 20 there-through. The inner sleeve 19 is also formed with anaperture 21 which has a curved edge 22. As is known in the art, a spring(not shown) biases the valve to the closed position in the absence of asufficient pressure difference across the spring.

As pressure across the valve 14 increases, the inner sleeve 19 movesfurther into the outer sleeve 18, and the apertures 20,21 overlap to agreater extent. A greater fluid volume can flow through the valve withincreased overlap of the sleeves. The volumetric flow rate (V) comparedwith pressure difference (dP) across the valve is depicted in FIG. 4.

In use, before the multiple stage pump assembly is installed in a well,the overall pressure increase to be obtained by the multiple stage pumpassembly is divided by the number of pumps in the series to obtain thethreshold pressure of the pressure controlled valves 14. The thresholdpressure of the valves is then set to this value. Alternatively, thethreshold pressure is set slightly above the value calculated. Forexample, if the required pressure increase for this multiple stage pumpassembly comprising four pump stages is 2000 psi (13.8 MPa), then thethreshold pressure for each pressure controlled valve 14 can be set to550 psi (3.79 MPa) (i.e. slightly above 2000/4). The pump can then beinstalled in the well.

In situations where the fluid in the well is all liquid, the pumpoperates as a conventional twin screw multiple stage pump assembly.Specifically, the liquid is pressurised equally at each stage and so thepressure difference across each pump stage is about 500 psi (3.45 MPa).The valves do not, therefore, open.

However, where the fluid comprises gas, the last pump in the seriesbegins to perform more work than the other pumps and the pressuredifference across that pump increases. If the pressure difference acrossthe last pump is greater than the threshold pressure of the pressurecontrolled valve 14, then the valve 14 will open and fluid, primarilyliquid, will be re-circulated from the outlet 8 of the pump through theconduit 11 and to the inlet 7 of the last pump.

By re-circulating liquid back to the inlet of the last pump, thepressure difference across the third pump is increased. Since the flowrate of the third pump is unchanged, it can be seen from equation 1above that this means that the third pump assembly is caused to workharder (increased power). Additionally, the increase in pressuredifference across the third pump causes the valve of the third valve toopen, permitting liquid to be re-circulated back to the inlet of thethird pump.

In turn, the valve of the second pump is caused to open and re-circulateliquid to the inlet of the second pump.

Consequently, each of the third, second and first pumps are forced towork harder and contribute more effectively to the pressure boostobtained by the multiple stage pump assembly.

It will be understood that the pressure difference across the first pump2 will also increase. However, since, in this embodiment, the first pump2 is a conventional twin screw pump, the pump will simply be forced towork harder.

In practice, the valves 14 of each of the last, third and second pumps 3open quickly, one after another, to varying degrees to allow liquid tore-circulate across or around the pumps establishing an equilibriumpressure distribution. If the gas to liquid ratio increases over time,the required volume differences between the pumps 3 will increasecausing the valves 14 to open further, permitting a greater volume ofliquid to be re-circulated (see FIG. 2).

It can be seen, therefore, that the pump assembly of the inventionautomatically regulates the opening of the valves to evenly distributethe work done by each pump in the assembly. Further, the pump assemblyautomatically and continuously responds to variations in the fluidcomposition being pumped.

In another embodiment, the first pump in the series can also be inaccordance with the first aspect of the invention. In this case, liquidcan be re-circulated from the outlet to the inlet of the first pump,thereby controlling the pressure difference across, and therefore workdone by, the first pump. Whilst this may ensure longevity of the firstpump, it will control the maximum power which the multiple stage pumpassembly can achieve.

FIG. 5 illustrates another valve which may be used in the invention. Thevalve 14 of FIG. 5 comprises a piston 23 having an inlet face 24 and anoutlet face 25. The inlet face 24 is the face which is exposed, in use,to the pump inlet pressure and the outlet face 25 is the face which isexposed, in use, to the pump outlet pressure. The surface area of theinlet face 24 is greater than the surface area of the outlet face 25. Apassage 26 extends through the piston to permit fluid flow through thevalve. The exit 27 of the passage 26 can be shaped to permit a varyingfluid flow rate, similar to the aperture 21 in FIG. 2.

It will be appreciated that the piston 23 in FIG. 5 acts as an actuatorto control the opening of the passage 26. Since the passage extendsthrough the piston, the actuator is integral with that part of the valvewhich provides a fluid flow path (the valve element). However, it ispossible for the actuator to be remote from that part of the valve whichprovides the fluid flow path whilst still actuating and controlling it.

The pressure controlled valve 14 responds to the ratio between thepressure at the pump outlet (which is acting on the outlet face 25 ofthe piston) and the pressure at the pump inlet (which is acting on theinlet face 24 of the piston). When the ratio between the pressure at thepump outlet and the pressure at the pump inlet reaches a threshold, thevalve permits fluid to flow there-through. The threshold corresponds tothe ratio between the surface area of the inlet face 24 to the surfacearea of the outlet face 25.

The ratio between the surface area of the inlet face 24 to the surfacearea of the outlet face 25 decreases from the first pump to the lastpump in the series, so that approximately the same pressure can be addedby each pump stage. For example, if it desired that each pump stageshould increase the fluid pressure by about 500 psi (3.45 MPa) and thebottom hole pressure is thought to be about 750 psi (5.17 MPa), theratio between the surface area of the inlet face 24 to the surface areaof the outlet face 25 for the first pump stage is about 1.67; for thesecond pump stage the ratio is about 1.4; for the third pump stage theratio is about 1.29; and for the last pump stage the ratio is about1.22.

Using valves of this type, the overall pumping pressure that can beobtained by the multiple stage pump assembly is not limited in the waymentioned above when each pump includes a valve of the type depicted inFIG. 2.

Yet another example of a valve 14 which can be used in the presentinvention is depicted in FIG. 6. This valve comprises a piston 28 havingend faces 29, a shaft 30 and two chambers 31,32. One of the chambers 31is in fluid communication with the intake 4 of the multiple stage pumpassembly 1 and the other chamber 32 is in communication with thedischarge 5 of the multiple stage pump assembly 1. Ports 34 through thevalve side wall allow the chambers 31,32 to be put in fluidcommunication with the intake 4 and discharge 5 of the multiple stagepump assembly 1.

It can be understood from the figure that the chambers 31, 32 areannular shaped around the shaft 33 of the piston 28. It will be furtherunderstood that the pressure in chamber 31 which corresponds to theintake pressure of the multiple stage pump assembly opposes the inletpressure of the pump stage. Similarly, the pressure in chamber 32 whichcorresponds to the discharge pressure of the multiple stage pumpassembly opposes the outlet pressure of the pump stage.

As with the valve shown in FIG. 5, the valve may alternatively bestructured such that the piston is remote from the fluid flow path.

The ratio of the surface area of the end faces 29 of the piston to thecross-sectional area of the chambers 31,32 prescribes a threshold ratio.When the ratio of the pressure difference between the outlet and theinlet of the pump stage (dP_(stage)) and the pressure difference betweenthe discharge and intake of the overall multiple stage pump assembly(dP_(assembly)) reaches the threshold ratio, the valve will permit fluidflow there-through.

To set the ratio for a multiple stage pump assembly comprising “n”pumps, the ratio of the surface area of the end faces 29 to thecross-sectional area of the chambers 31,32 is n:1. Accordingly, in amultiple stage pump assembly such as that shown in FIG. 1 which has 4pump stages, the surface area of the end faces 29 of the piston 28should be about four times the cross-sectional area of the chambers31,32.

For a valve with a given piston end face 29 surface area, the ratiobetween the end face 29 surface area and the chamber 31,32cross-sectional area can be varied by varying the diameter of thepiston's shaft 30.

With such an arrangement, it is possible to distribute the work acrossall the pumps without knowing what the bottom hole pressure is. Althoughthe chambers 31, 32 have been described as annular, and this isadvantageous chambers of other shapes may also be used. The function ofthe chambers 31, 32 is to enable the valve of FIG. 6 to be actuatedbased on the ratio of the pressure difference across an individual pumpto the pressure difference of the multi-pump assembly as a whole. Forexample, in a multiple stage pump assembly comprising a plurality ofindividual pumps arranged in series, the inlet of an individual pump maybe coupled to the outlet of that individual pump by a fluid bypassarranged to enable recirculation of fluid from the outlet of thatindividual pump to its inlet. The fluid bypass typically comprises acontrol valve, configured to control the recirculation based on thepressure drop across the individual pump, e.g. the pressure differencebetween the outlet of that individual pump and its inlet. The controlvalve may also be controlled based on the pressure between the inlet ofthe multiple stage pump and the outlet of the multiple stage pump. Thisenables, for example the control valve to control recirculation throughthe fluid bypass of an individual pump based on the ratio of thepressure drop across the individual pump to the pressure drop across themultiple stage pump assembly. This may be achieved as described above byproviding fluid couplings into the control valve from the outlet/inletof the multiple stage pump assembly or by other means for example byelectronic control of the control valves.

FIG. 7 shows another example of a multiple stage twin screw pumpassembly similar to that shown in FIG. 1, and so like numerals refer tolike parts. The multiple stage twin screw pump assembly shown in FIG. 7is made up of four pumps. Each pump is a conventional twin screw pump 2.The second, third and fourth pumps each further comprise an inlet 40 andan outlet 41 adaptor which are connected to each other via a conduit 42,such as a pipe. It can be seen that the conduits 42 are external to theconventional twin screw pumps 2. A pressure controlled valve 14 ispositioned in each conduit 42, though it could also be positioned at theinlet or outlet to the conduit 42.

Accordingly, it can be seen that a conventional twin screw pump can beused to make a pump in accordance with the present invention.

The inlet/outlet adaptors 40,41 are units which can be connected to theinlet/outlet 7,8 of the conventional twin screw pump and which have achamber for containing the fluid. Fluid is discharged from the outlet ofa conventional pump 2 into the adjacent outlet adaptor 41 so that it canbe passed on to the next pump assembly in the series. According to theinvention, some of the fluid can be re-circulated to the inlet adaptor40 when the pressures across the conventional twin screw pump 2 causethe valve to open. The valve can be any of the valves described above.The conduits 42 are connected to the chambers inside the respectiveoutlet adaptors 41 near the bottom so that the chambers can act as smallseparation tanks, thereby enabling liquid to be preferentiallyre-circulated to the inlet adaptors 40.

In this way, a multiple stage pump assembly can be constructed usingconventional rotary screw pumps.

FIG. 8 shows another embodiment of the invention in which conventionalrotary screw pumps are used to form pumps and a multiple stage pumpassembly in accordance with the invention. Again, like referencenumerals refer to like parts.

In this embodiment, rather than providing the conventional pumps withinlet and outlet adaptors adjacent the inlets and outlets of the second,third and fourth conventional pumps, only outlet adaptors 45 areprovided. An outlet adaptor 45 is coupled to the outlet 8 of each of theconventional twin screw pumps 2 so that fluid is delivered from the pumpto a chamber inside the outlet adaptor.

Each outlet adaptor 45 is also connected to the outlet adaptor 45 of theadjacent pump assemblies via a conduit 46. As can be seen from thefigure, the conduit 46 is a single conduit with a connection point 47for each outlet adaptor 45. Pressure controlled valves 14 are positionedin the conduit 46 to separate each connection point.

Where the fluid being pumped is 100% liquid, the valves 14 remainclosed.

However, as in the first example described above with respect to FIG. 1,if gas is present in the fluid, the pressure difference across the lastpump will increase, causing the valve 14 located between the outletadaptors 45 of the fourth and third pump assemblies to open. Fluid willflow from the outlet adaptor 45 of the last pump assembly and into theconduit 46. Since the outlet adaptor 45 of the third pump assembly is influid communication with the inlet of the fourth pump assembly, thepressure in that outlet adaptor is lower than the pressure of the fluidbeing re-circulated in the conduit 46 and so the fluid will flow intothe outlet adaptor of the third pump assembly.

In turn, the pressure difference across the third pump in the seriesincreases and the corresponding valve opens to re-circulate liquid, andso on for the second and first pumps. In practice, the valves open andreach equilibrium almost instantly.

It can be understood that this arrangement of outlet adaptors 45, valves14 and the conduit 46 can be used with conventional twin screw pumps toform pumps and a multiple stage pump assembly in accordance with theinvention.

In an alternative arrangement, the conduit 46 may not be a singleconduit. There may instead be separate conduits connecting adjacentoutlet adaptors 45. In that case, the outlet adaptors connected to theoutlets of the second and third pump assemblies each have two conduitsconnected thereto; one which feeds pressurised fluid into the outletadaptor and one which takes fluid away for re-circulation.

FIG. 9 shows a multiple stage twin screw pump assembly which is tapered.The pump assembly comprises four pumps of decreasing swept volume fromthe intake end 4 to the discharge end 5. The decreasing swept volume maybe achieved as is well known in the art. For example, the pitch of thethreads on the rotors may decrease from the intake end to the dischargeend.

Each pump is constructed in accordance with the first aspect of theinvention, in that it has a conduit 11 and pressure controlled valve 14to selectively allow re-circulation of fluid from the outlet to theinlet of the respective pump. Accordingly, these pumps are similar tothose described above with respect to FIG. 1, except that they form atapered pump assembly. Accordingly, like reference numerals refer tolike parts.

In use, it is well known in the industry that a tapered pump can bedesigned specifically for a particular gas to liquid ratio. Accordingly,the swept volume of each of the four pumps is selected as is known tothe skilled reader so that the multiple stage pump can handle apredefined gas to liquid ratio. If, in use, the gas to liquid ratio ofthe fluid encountered increases above the predefined ratio, the pumpoperates in the same way as described above with reference to FIG. 1.Specifically, the valves open and re-circulate fluid to the respectivepump inlets.

If the gas to liquid ratio decreases below the predefined ratio, thenthe first pump delivers too much fluid to the second pump, the secondpump delivers too much fluid to the third assembly and so on. Thepressure differences across the pumps therefore increase and so thevalves open and re-circulate liquid from the respective outlets to therespective inlets. However, in contrast to the discussion above, in thissituation, the valve of the first pump reacts first, followed by thevalves of the subsequent pumps. Again, though, successive opening of thevalves is, in practice, relatively quick.

An example of where this embodiment can be useful is where a well hasbeen killed by injecting heavy “kill fluid” (primarily liquid) into awell. It may be known that the well typically produces a fluid with aparticular gas to liquid ratio. A tapered multiple stage pump assemblyin accordance with the invention can be tailored for that gas to liquidratio. Although the pump is optimised for the normal composition of thewell fluid, it is still able to pump the heavy kill fluid out of thewell when it is desired to put the well back into operation, since fluidcan be re-circulated as described above. Specifically, for the periodwhen the kill fluid is to be pumped out, the gas to liquid ratio islower than the ratio for which the pump is tailored. Too much fluid isdelivered to the subsequent pumps. Liquid would be re-circulatedinitially from the outlet to the inlet of the first pump and then ofsubsequent pumps in the series.

It can be seen that a tapered pump as described above can efficientlypump a wide variety of gas to liquid ratios.

It is to be understood that features described above with reference toone of the embodiments may be used in conjunction with otherembodiments. Also, variations will be apparent to the skilled reader,for example the tapered pump shown in FIG. 9 may comprise a conventionaltwin screw pump in the first or last stage instead of the pump of theinvention, so as to handle more or less gas respectively than the gas toliquid ratio for which the tapered pump is designed. Also, any of thedescribed valves can be used in any of the embodiments of pump assembly.

The invention claimed is:
 1. A multiple stage pump assembly comprisingat least two pumps arranged in series, wherein at least the second andeach subsequent pump comprises a pump inlet, a pump outlet, at least twothreaded rotors and a pressure controlled valve, the pressure controlledvalve being adapted to control re-circulation of fluid from the pumpoutlet to the pump inlet, said pressure controlled valve being one whichresponds to a ratio between the pressure at the pump outlet and thepressure at the pump inlet, such that the valve permits fluid to flowthere-through when the ratio between the pressure at the pump outlet andthe pressure at the pump inlet reaches a threshold, said pressurecontrolled valve comprising a piston having an inlet face and an outletface, a surface area of the inlet face being greater than a surface areaof the outlet face and a ratio between the area of the inlet face to thearea of the outlet face prescribing the threshold ratio between thepressure at the pump outlet and the pressure at the pump inlet.
 2. Theassembly of claim 1, in which the pressure controlled valve is adaptedto control the rate of fluid flow there-through in proportion to the gasto liquid ratio of a fluid being pumped by the pump in use.
 3. Theassembly of claim 1, in which the pressure controlled valve is a controlvalve.
 4. The assembly of claim 1, further comprising a conduitconnecting the pump outlet to the pump inlet for flowing re-circulatedfluid from the pump outlet to the pump inlet.
 5. The assembly of claim4, in which the pressure controlled valve is located wholly or partlywithin the conduit or adjacent one or other end of the conduit.
 6. Theassembly of claim 4, further comprising a recess in the pump outlet toallow, in use, liquid rather than gas to flow from an enclosure in whichthe threaded rotors are located to the conduit.
 7. The assembly of claim1, wherein each pump has the same swept volume.
 8. The assembly of claim1, wherein there is a reduction in the swept volume of each pump fromthe first pump to the last pump in the series.
 9. A method of pumping afluid from a first location to a second location comprising providing amultiple stage pump assembly having two or more pumps in series whereinthe second and each subsequent pump is adapted to re-circulate fluidfrom its outlet to its inlet, positioning an intake end of the multiplestage pump assembly at or near the first location, activating themultiple stage pump assembly to pump the fluid from the first locationto the second location, and re-circulating fluid from the outlet to theinlet of said second and each subsequent pump, said pressure controlledvalve being one which responds to a ratio between the pressure at thepump outlet and the pressure at the pump inlet, such that the valvepermits fluid to flow there-through when the ratio between the pressureat the pump outlet and the pressure at the pump inlet reaches athreshold, said pressure controlled valve comprising a piston having aninlet face and an outlet face, a surface area of the inlet face beinggreater than a surface area of the outlet face and a ratio between thearea of the inlet face to the area of the outlet face prescribing thethreshold ratio between the pressure at the pump outlet and the pressureat the pump inlet.
 10. A multiple stage pump assembly comprising atleast two pumps arranged in series, wherein at least the second and eachsubsequent pump comprises a pump inlet, a pump outlet, at least twothreaded rotors and a pressure controlled valve, the pressure controlledvalve being adapted to control re-circulation of fluid from the pumpoutlet to the pump inlet, wherein the pressure controlled valve is onewhich responds to a ratio between the pressure difference between theoutlet and the inlet of the pump (dP_(stage)) and the pressuredifference between first and second pressures (dP_(assembly)) which, inuse, are communicated to the valve, such that the valve permits fluid toflow there-through when the ratio between dP_(stage) and dP_(assembly)reaches a threshold.
 11. The assembly of claim 10, in which the valvecomprises a piston having end faces and two chambers, one chamber beingadapted for fluid communication with an intake of a multiple stage pumpassembly which is at the second pressure and the other chamber beingadapted for fluid communication with a discharge of said multiple stagepump assembly which is at the first pressure, such that, in use, thepressures in the chambers which correspond to the intake and dischargepressures of the multiple stage pump assembly oppose the inlet andoutlet pressures of the pump respectively, wherein a ratio of a surfacearea of the end faces to a cross-sectional area of the chambersprescribes the threshold ratio at which the valve will permit fluidflow.
 12. The assembly of claim 10, in which the pressure controlledvalve is adapted to control the rate of fluid flow there-through inproportion to the gas to liquid ratio of a fluid being pumped by thepump in use.
 13. The assembly of claim 10, in which the pressurecontrolled valve is a control valve.
 14. The assembly of claim 10,further comprising a conduit connecting the pump outlet to the pumpinlet for flowing re-circulated fluid from the pump outlet to the pumpinlet.
 15. The assembly of claim 14, in which the pressure controlledvalve is located wholly or partly within the conduit or adjacent one orother end of the conduit.
 16. The assembly of claim 14, furthercomprising a recess in the pump outlet to allow, in use, liquid ratherthan gas to flow from an enclosure in which the threaded rotors arelocated to the conduit.
 17. The assembly of claim 10, wherein each pumphas the same swept volume.
 18. The assembly of claim 10, wherein thereis a reduction in the swept volume of each pump from the first pump tothe last pump in the series.